This invention relates generally to alternator systems and more particularly to alternator systems used in vehicles.
As is known in the art, an alternator is an alternating current (ac) output generator. To convert the ac voltage to direct current (dc) for use in charging batteries or supplying dc loads, for example, a rectifier system is used. Sometimes, the alternator is referred to as an ac machine or more simply a machine and the combined generator/rectifier system is referred to as an alternator or an alternator system.
In many cases (including automotive alternators), a diode rectifier is used to rectify the ac voltage produced by the generator. The ac generator can be modeled as a three-phase voltage source and a set of inductors.
In a so-called wound-field machine, the output voltage or current can be controlled by varying the current in a field winding which in turn varies the ac voltage magnitudes. The advantage to this approach is the extreme simplicity and low cost of the system. One particular type of wound field machine is a so-called wound-field Lundell-type alternator. A lundell machine is characterized by the way the rotor/field of the machine is constructed, the details of which are well-known to those of ordinary skill in the art. Significantly, the construction techniques used to manufacture Lundell-type result in an ac machine which is relatively inexpensive but which has a relatively high leakage inductance or reactance. Wound-field Lundell-type alternators are almost universally used in the automotive industry primarily because they are reliable and inexpensive. One problem with wound-field Lundell-type alternators, however, is that the relatively high machine inductance strongly affects the machine performance. In particular, due to the high inductance of the Lundell machine, it exhibits heavy load regulation when used with a diode rectifier. That is, there are significant voltage drops across the machine inductances when current is drawn from the machine, and these drops increase with increasing output current and machine operating speed. Consequently, to deliver substantial current into a low dc output voltage, the ac machine voltage magnitudes have to be much larger than the dc output voltage.
For example, in a typical high-inductance automotive alternator operating at relatively high speed, the internal machine voltage magnitudes are in excess of 80 V to deliver substantial current into a 14 V dc output. This is in contrast with a low-reactance machine with a diode rectifier, in which the dc output voltage is only slightly smaller than the ac voltage magnitudes.
In order to control output voltage or current, a controlled rectifier is sometimes used instead of field control. One simple and often-used approach for controlled rectification is to replace the diodes of a diode rectifier with thyristor devices. For example, as described in J. Schaefer, Rectifier Circuits, Theory and Design, New York: Wiley, 1965 and J. G. Kassakian, M. F. Schlecht, and G. C. Verghese, Principles of Power Electronics, New York: Addison-Wesley, 1991 thyristor devices can be used in a semi-bridge converter. With this technique, phase control (i.e. the timing of thyristor turn on with respect to the ac voltage waveform) is used to regulate the output voltage or current. One problem with this approach, however, is that it can be relatively complex from a control point of view. This is especially true when the alternator must provide a constant-voltage output.
Alternatively, rather then using phase control, control is sometimes achieved using switched-mode rectification. With the switched-mode rectification technique, fully-controllable switches are used in a pulse width modulation (PWM) fashion to produce a controlled dc output voltage from the ac input voltage. This approach, which typically utilizes a full-bridge converter circuit, often yields high performance at the expense of many fully-controlled PWM switches and complex control circuits and techniques.
One relatively simple switched-mode rectifier that has been employed for alternators attached to wind turbines is described in an article entitled xe2x80x9cVariable Speed Operation of Permanent Magnet Alternator Wind Turbines Using a Single Switch Power Converter,xe2x80x9d by G. Venkataramanan, B. Milkovska, V. Gerez, and H. Nehrir, Journal of Solar Energy Engineeringxe2x80x94Transactions of the ASME, Vol. 118, No. 4, November 1996, pp. 235-238. In this approach, the alternator includes a rectifier comprising a diode bridge followed by a xe2x80x9cboost switch setxe2x80x9d provided from a controlled switch (such as a MOSFET) and a diode. The switch is turned on and off at a relatively high frequency in a PWM fashion. This approach is utilized along with PWM switching generated by a current-control loop to simultaneously control the output current and turbine tip speed of a permanent magnet alternator. The approach is specifically applied to a low-reactance (i.e. low-inductance) permanent-magnet ac machine where the battery voltage is higher than the ac voltage waveform. It should be noted that the rectifier system is topologically the same as the Discontinuous Conduction Mode (DCM) rectifier described in an article entitled xe2x80x9cAn Active Power Factor Correction Technique for Three-Phase Diode Rectifiers,xe2x80x9d by A. R. Prasad, P. D. Ziogas, and S. Manias, the IEEE Trans. Power Electronics, Vol. 6, No. 1, January 1991, pp. 83-92, but the operating mode and control characteristics of the single switch power converter and DCM rectifier are very different.
Another controlled rectifier approach for alternators is described in U.S. Pat. No. 5,793,625, issued Aug. 11, 1998. This patent describes a circuit which utilizes the application of boost mode regulator techniques to regulate the output of an ac source.
The source inductance becomes part of the boost mode circuit, thus avoiding the losses associated with the addition of external inductors. When a three-phase alternator is the power source, the circuit comprises a six diode, three-phase rectifier bridge, three field effect transistors (FETs) and a decoupling capacitor. The three FETs provide a short circuit impedance across the output of the power source to allow storage of energy within the source inductance. During this time, the decoupling capacitor supports the load. When the short circuit is removed, the energy stored in the inductances is delivered to the load. Because the circuit uses the integral magnetics of the ac source to provide the step-up function, a relatively efficient circuit is provided. The duty cycle of the switches (operated together) is used to regulate the alternator output voltage or current. The rectifier can thus be used to regulate the output voltage and improve the current waveforms for low-reactance machines that would otherwise operate with discontinuous phase currents.
While regulating output voltage or current with a boost circuit of this type may be useful in permanent magnet alternators having relatively low inductance characteristics, this method is not useful with alternators having a relatively large inductance characteristic and a wide operating speed range such as in wound-field Lundell-type alternators for automotive applications.
To understand this, consider that in a system which includes an alternator coupled to a boost rectifier, the output voltage is fully controllable by the boost rectifier when the internal machine voltages are the same magnitude or lower than the dc output voltage as described, for example, in the above referenced Venkataramanan paper. However, if the internal machine voltages become significantly larger than the desired dc output voltage, then the output voltage cannot be regulated by the boost rectifier independent of load without inducing unacceptably high currents in the machine. The typical automotive Lundell alternator presents this problem.
At the present, high-reactance Lundell-type alternators with diode rectifiers and field control are widely used in the automotive industry. Moreover, there is a very large infrastructure dedicated to the manufacture of Lundell-type alternators. However, design of these alternators is becoming increasingly more difficult due to continually rising power levels required in vehicles and in particular required in automobiles.
As is also known, the average electrical load in automobiles has been continuously increasing for many years. The increase in electrical load is due to the demand to provide automobiles and other vehicles with increasingly more electronics and power consuming devices such as microprocessors, electric windows and locks, electromechanical valves, and electrical outlets for cell phones, laptop computers and other devices. Such additional electronics results in a need for more electrical energy in automobiles and other vehicles.
Because of this increase in electrical load, higher power demands are being placed on automotive alternator systems. Furthermore, the increasing power levels have motivated the adoption of a new higher distribution voltage in automobiles to augment and/or replace the current 14 V distribution system. In some cases, a single high-voltage electrical system may likely be used (e.g. a 42 volt electrical system). In other cases, a dual-voltage electrical system may be used which includes a first relatively high-voltage system (e.g. a 42 V electrical system) and a second relatively low-voltage system (e.g. a 14 V electrical system.) The high-voltage electrical system will be used to power vehicle components which require a relatively large amount of power such as a starter motor of a vehicle. When retained (in the dual-voltage case), the low-voltage system will be used to power vehicle components that benefit from a low-voltage supply such as incandescent lamps and signal-level electronics.
A dual- or high-voltage system having a starter motor coupled to a high-voltage bus requires a charged high-voltage battery to start. In cases where the high-voltage battery is not fully charged or is depleted, it would be desirable to be able to charge the depleted high-voltage battery from a low-voltage source in order to provide xe2x80x9cjump-startxe2x80x9d capability for dual/high voltage systems. In an automobile which includes only a single high-voltage system, one may desire to transfer energy from a low-voltage power source, battery or alternator of a different vehicle to the high-voltage system. In a dual-voltage system, one may desire to transfer energy from a low-voltage battery of the dual-voltage system to the high voltage battery of the dual-voltage system or from the low-voltage battery or alternator of a different vehicle or other low-voltage source to the high voltage battery of the dual-voltage system.
It would, therefore, be desirable to provide a means by which the power output capability of an alternator can be increased. It would also be desirable to provide an alternator which is capable of efficient operation at a plurality of different voltage levels.
It would be further desirable to provide an alternator which is capable of operating in a dual voltage automobile. It would be still further desirable to provide a system and technique for transferring energy from a low-voltage battery of a first vehicle or from an alternator of a second different vehicle or other source to a high-voltage bus of the first vehicle.
In accordance with the present invention, a system for charging a battery at the output of the switched-mode rectifier includes means for selectively connecting the positive terminal of a charging source (which may be a low-voltage source) to the machine neutral point. This may be done via a connector or a switch (such as a mechanical switch, relay, or semiconductor switch), or by another similar means. The negative terminal of the charging source is connected to system ground (as is the negative terminal of the high-voltage battery). In this configuration the ac machine inductances in conjunction with the switched-mode rectifier can be used as a dc/dc converter to charge the high-voltage battery from the charging source. In one embodiment, the switched mode rectifier includes a plurality of metal oxide semiconductor field effect transistors (MOSFETs). When the MOSFETs are turned on, the current in the ac machine inductances increases, drawing energy from the low-voltage charging source and storing it in the machine inductances. When the MOSFETs are turned off, some of this energy plus additional energy from the charging source are transferred to the high-voltage battery through the diodes. The high-voltage battery may be charged from a low-voltage charging source (for jump-starting purposes, for example) using this method.
It should be recognized that this approach may be utilized in dual-voltage systems as well. In the case of a dual-voltage system, the charging source may be the low-voltage battery of the same vehicle, or it may be supplied from a different vehicle or source. Again, a means is provided for selectively connecting a neutral of the ac machine to the desired charging source. In a dual-voltage system charging from its own low-voltage battery, this connection may be conveniently provided by a relay connecting the machine neutral to the positive terminal of the low-voltage battery, for example.
In accordance with a further aspect of the present invention, a method for charging a battery at an output of a switched-mode rectifier includes the steps of connecting the positive terminal of a charging source to a neutral point of an ac machine, connecting the negative terminal of the charging source to system ground (it is assumed that the negative terminal of the high voltage source is already connected to ground), increasing the current in the ac machine to draw energy from the charging source and storing the energy in the machine inductances and transferring the energy plus additional energy from the charging source to the high-voltage battery. With this method, the high-voltage battery may be charged from a low-voltage source for jump-starting purposes, for example.
The connection between the positive terminal of the low voltage source and the ac machine neutral point may be provided via a connector or a switch (such as a mechanical switch, relay, or semiconductor switch), or by another similar means. The current in the ac machine may be increased to draw energy from the low-voltage source by providing a low impedance path between the output terminals of the alternator and system ground. This may be accomplished, for example, by turning on switches in a switched mode rectifier coupled to the ac machine.
In this configuration, the ac machine inductances in conjunction with the switched-mode rectifier can be used as a dc/dc converter to charge the high-voltage battery from the charging source. When the switches of the switched-mode rectifier, which may be provided as MOSFETs for example, are turned on, the current in the machine inductances increases, drawing energy from the low-voltage source and storing it in the machine inductances. When the switches are turned off, some of this energy plus additional energy from the charging source are transferred to the high-voltage battery through the diodes.
It should be recognized that this approach may be utilized in dual-voltage systems as well. In the case of a dual-voltage system, the charging source may be the low-voltage battery of the same vehicle, or it may be supplied from a different vehicle or source. Again, a means is provided for selectively connecting the alternator machine neutral to the desired charging source. In a dual-voltage system charging from its own low-voltage battery, this connection may be conveniently provided by a relay connecting the machine neutral to the positive terminal of the low-voltage battery, for example.
It should also be recognized that this approach may be used in systems employing rectifier types other than a boost rectifier, such as those described below. With other rectifier types, the charging source voltage may in fact be the same or larger in magnitude than the high-voltage battery at the switched-mode rectifier output. It should also be recognized that connection points other than the machine neutral may be used with this approach. Again, in these cases, the alternator inductances and switched-mode rectifier may be used as a dc/dc converter to provide controlled charging from the charging source to the high-voltage battery.
In accordance with a still further aspect of the present invention, an alternator system having an alternating current (ac) voltage source controlled by a field current regulator and an internal inductance includes a switched-mode rectifier coupled to the ac voltage source, a switched-mode rectifier (SMR) control circuit coupled to the switched-mode rectifier and a speed sensor coupled to the SMR control circuit.
With this particular arrangement, an alternator system capable of increased alternator system power output is provided. The speed sensor senses a frequency or operating speed of the ac voltage source and provides a signal representative of the frequency or speed to the SMR control circuit. In response to the frequency or speed information provided thereto, the SMR control circuit provides a duty ratio signal to the switched-mode rectifier which causes the switched-mode rectifier to operate with a particular duty cycle. The switched-mode rectifier duty cycle is thus selected based upon the frequency or speed of the ac voltage source. When the switched-mode rectifier is operated in this manner, the alternator system achieves levels of power and performance that are higher than those conventionally achieved.
The switched-mode rectifier operates at a duty cycle selected to provide a controlled transformation of voltage and current between terminals of the ac voltage source and output terminals of the alternator system and converts an ac voltage from the ac voltage source to a direct current (DC) voltage. In this manner the switched-mode rectifier transforms the voltage at the output of the ac voltage source such that the switched-mode rectifier stage extracts relatively high levels of power and performance from the ac voltage source.
Because of the load regulation in a high reactance machine, for maximum field current, the output power that the ac machine delivers is a function of the speed of the machine and the effective voltage seen by the ac machine. For a given speed, there is a single effective alternator voltage at which maximum power output will be achieved (as illustrated in the curves of FIG. 3 discussed below). The switched-mode rectifier provides a controlled transformation of voltage and current between the terminals of the alternator machine and the alternator system output. The transformation is controlled by a duty ratio d. In the case where the switched-mode controller includes a boost rectifier circuit, the machine sees a local equivalent voltage vx that is (1xe2x88x92d) times the output voltage, and the output receives a current that is (1xe2x88x92d) times the machine current, so that the boost rectifier appears as an ideal transformer with turns ratio 1xe2x88x92d. By controlling the duty ratio of the switched-mode rectifier as a function of speed (rpm) one can ensure that the alternator machine can always achieve its maximum power output independent of the fixed alternator system output voltage. It should be appreciated that the speed sensor can sense any parameter or combination of parameters related to ac machine speed (e.g. engine speed, frequency, alternator speed, frequency, alternator back emf, etc.) and provide an appropriate signal to the SMR control circuit.
At the same time the present invention provides a circuit that is relatively simple and inexpensive. The alternator system can also include a field controller and a field current regulator coupled to the ac voltage source that controls the ac voltage source magnitude. The field control can be used as a primary means for regulating output voltage or current in the wound-field alternator. The switched-mode rectifier stage is controlled and acts as a second control handle to extract relatively high levels of power and performance from the alternator.
In one embodiment the alternator system can optionally control the switched-mode rectifier duty ratio as a function of both the alternator speed and the field current magnitude. To achieve maximum power from the machine (at full field current) it is sufficient to control the duty ratio as a function of speed. By controlling the duty ratio as a function of both speed and field current, it is possible to achieve improved operation (e.g. higher efficiency) at partial load in addition to the improvement in maximum output power. It should be appreciated that the field current can be determined by any parameter or combination of parameters related to field current, e.g. field current, average field voltage, field controller duty ratio, alternator back emf, field winding magnetic field strength, etc. It should also be appreciated that the switched-mode rectifier duty ratio can be controlled based on measurements related to joint functions of field current and speed, such as alternator back emf (which is related to the product of alternator speed and field current).
In a different embodiment, the alternator system can optionally control the switched mode rectifier duty ratio as a f-unction of the machine back emf, which is related to the product of alternator speed and field current. Controlling the switched-mode rectifier as a function of machine back emf will ensure that the alternator machine can always achieve its maximum power output independent of the fixed alternator system output voltage, and will also provide high efficiency operation at output power levels below maximum. A machine back emf sensor (which may comprise a sense winding and processing electronics or other means of measuring or estimating the back emf) may be provided in place of or in addition to the speed sensor in this embodiment.
In one embodiment, the alternator system can optionally include a fault protection controller coupled to the SMR control circuit. The fault protection controller operates under fault conditions (e.g. load dump), and overrides the other controllers in the alternator system based on output voltage when a load dump occurs.
In operation, in response to detection of a fault condition by the fault protection controller, (e.g. when a significant over-voltage is detected at the output terminals of the alternator system) the fault protection controller overrides both the field and switched-mode rectifier controllers such that the field current is driven down and the switched-mode rectifier limits the load dump transient at the output. In this manner, the fault protection controller provides a means for implementing load dump protection. Thus, inclusion of the fault protection controller provides an alternator system having a greater degree of circuit protection than can be achieved with a conventional diode rectifier.
In one embodiment the alternator system can optionally include a connecting system for selectively connecting the positive terminal of a charging source to the machine neutral point, along with a jump charging controller coupled to the SMR control circuit. This enables the switched-mode rectifier to be used in conjunction with the alternator machine inductances as a dc/dc converter to charge the battery at the output of the switched-mode rectifier from the charging source. The inclusion of these elements provides an important improvement in the alternator system functionality over what is achieved in conventional systems.
The circuit of the present invention is well suited to use with high-reactance wound-field alternators, including automotive Lundell-type alternators and therefore finds immediate applicability in use with automotive alternators. The present invention also finds use in any application which requires an alternator including but not limited to the petroleum exploration industry, where a downhole alternator, connected to a turbine driven by drilling mud, is used as a downhole power source in directional drilling operations. The invention also finds use in generators for marine and aerospace applications, portable generators and backup power supplies.
With the present invention, relatively high power levels can be achieved within the existing manufacturing framework and with existing machine sizes at relatively low cost. Furthermore, the so-called load dump problem associated with Lundell and other wound field types of alternators is overcome by the addition of some control circuitry (e.g. a fault protection controller coupled to sense voltage levels at the output or at other locations of the alternator system), a relatively small change in the rectifier stage (e.g. coupling of the rectifier stage to the fault protection controller) and minor adjustments in the machine design so that the peak of the machine""s output power versus output voltage curve for constant speed with diode rectification matches the desired output voltage at a desired cruising speed, rather than at idle.
Changes to the machine, for example, could be implemented by providing the ac machine having a particular number of turns and a particular wire gauge selected such that at a desired cruising speed the peak of the ac machine""s output power vs. dc output voltage curve for diode rectification occurs at the desired output voltage. Those of ordinary skill in the art will appreciate of course that other parameters of the ac machine in addition to or in place of the number of turns and wire gauge may also be appropriately selected to achieve desired operation of the ac machine. Thus, relatively simple modifications to the winding of a conventional alternator can be made for good operation in accordance with the present invention at any desired output voltage.
The steps to wind of the alternator for a desired voltage are as follows: first, select a suitable cruising speed to design for and second, choose the number of alternator stator winding turns such that the peak of the output power versus output voltage curve (for diode rectification) at the design speed reaches its maximum at the desired output voltage. It should be noted that there are other more sophisticated modifications that can be made to an alternator for good operation in accordance with the present invention, such as reoptimizing the magnetic and thermal design of the alternator.
In addition to trends towards higher power in automotive alternators, there will be a need for high-power automotive alternators which generate power at higher voltages (e.g. at a voltage of 42 volts (V) instead of 14 V). With the present invention, by changing only the rectifier stage and control circuits even present 14 V ac machine designs are suitable for high-power operation at 42 V output. That is, by merely replacing the diode rectifier stage of a conventional alternator system with the switched-mode rectifier of the present invention, changing the field controller to one that takes input from a fault or load-dump protection controller, and replacing the control circuitry with appropriate new controls as described above, present machines designed for operation at 14 V can operate at 42 V. It should be appreciated that with such modifications operation can be achieved at other voltages including but not limited to 42 V.
With such a relatively simple change, it may be possible to manufacture both 14 V and 42 V versions of an alternator on the same manufacturing line. Thus, the new invention is timely for meeting the demands of higher power and higher voltage alternators in the automotive industry while remaining within the existing manufacturing framework and overcoming the present day load dump transient problem.
It should be noted that in prior art systems employing a switched-mode rectifier, the rectifier is used to regulate the output voltage. This is in contrast to the operation of an alternator system which operates in accordance with the present invention in which field control is used to regulate the output voltage and the switched-mode rectifier functions to provide xe2x80x9cload matchingxe2x80x9d between the machine and the load so that much higher levels of power can be extracted from the alternator than could be achieved with a diode rectifier.
The differences between such prior art systems and the system of the present invention become clear when one considers the details of the control circuitry. In a prior-art system using a switched-mode rectifier, the alternator system output voltage (or current) is an input to the controller of the switched-mode rectifier. In accordance with the present invention, however the frequency or speed of the ac machine is provided to the controller of the switched-mode rectifier. Thus, the SMR need only utilize the frequency or speed of the ac machine to determine the duty ratio of the switched-mode controller. A field controller and a field current regulator or other output voltage control means coupled to the ac machine may be used to primarily regulate the output voltage.